Police were called to reports of "a concern for safety" after a skip wagon was spotted stationary, with its engine running, near the exit slip road at junction 25 of the M62 at Brighouse.
Officers found a man in his forties on the embankment nearby when they arrived at the scene at about 2pm yesterday.
He was treated by paramedics but pronounced dead, West Yorkshire Police confirmed today (Wednesday).
The force said the man's family have been informed.
Police are appealing anyone who saw what happened before they arrived to come forward.
They are also urging motorists to check dash camera footage.
A West Yorkshire Police spokesperson said: "Officers are working to determine the circumstances of what took place but there is not believed to be any third party involvement at this time."
Anyone with any information is asked to contact police on 101, quoting log 820 of January 2.